Electrochromism is a phenomenon in which a material exhibits a reversible electrochemically-mediated change in an optical property when placed in a different electronic state, typically by being subjected to a voltage change. The optical property is typically one or more of color, transmittance, absorbance, and reflectance. One well known electrochromic material, for example, is tungsten oxide (WO3). Tungsten oxide is a cathodic electrochromic material in which a coloration transition, transparent to blue, occurs by electrochemical reduction.
Electrochromic materials may be incorporated into, for example, windows and mirrors. The color, transmittance, absorbance, and/or reflectance of such windows and mirrors may be changed by inducing a change in the electrochromic material. One well known application of electrochromic materials, for example, is the rear view mirror in some cars. In these electrochromic rear view mirrors, the reflectivity of the mirror changes at night so that the headlights of other vehicles are not distracting to the driver.
While electrochromism was discovered in the 1960's, electrochromic devices still unfortunately suffer various problems and have not begun to realize their full commercial potential. Advancements in electrochromic technology, apparatus and related methods of making and/or using them, are needed.
A typical electrochromic device includes an electrochromic (“EC”) electrode layer and a counter electrode (“CE”) layer, separated by an ionically conductive (“IC”) layer that is highly conductive to ions and highly resistive to electrons. In other words, the ionically conductive layer permits transport of ions but blocks electronic current. As conventionally understood, the ionically conductive layer therefore prevents shorting between the electrochromic layer and the counter electrode layer. The ionically conductive layer allows the electrochromic and counter electrodes to hold a charge and thereby maintain their bleached or colored states. In conventional electrochromic devices, the components form a stack with the ion conducting layer sandwiched between the electrochromic electrode and the counter electrode. The boundaries between these three stack components are defined by abrupt changes in composition and/or microstructure. Thus, the devices have three distinct layers with two abrupt interfaces.
Quite surprisingly, the inventors have discovered that high quality electrochromic devices can be fabricated without depositing an ionically conducting electrically insulating layer. In accordance with certain embodiments, the counter electrode and electrochromic electrodes are formed immediately adjacent one another, often in direct contact, without separately depositing an ionically conducting layer. It is believed that various fabrication processes and/or physical or chemical mechanisms produce an interfacial region between contacting electrochromic and counter electrode layers, and this interfacial region serves at least some functions of an ionically conductive electronically insulating layer in conventional devices. Certain mechanisms that may be key to forming the interfacial region are described below.
The interfacial region typically, though not necessarily, has a heterogeneous structure that includes at least two discrete components represented by different phases and/or compositions. Further, the interfacial region may include a gradient in these two or more discrete components. The gradient may provide, for example, a variable composition, microstructure, resistivity, dopant concentration (for example, oxygen concentration), and/or stoichiometry.
In addition to the above discoveries, the inventors have observed that in order to improve device reliability, two layers of an electrochromic device, the electrochromic (EC) layer and the counter electrode (CE) layer, can each be fabricated to include defined amounts of lithium. Additionally, careful choice of materials and morphology and/or microstructure of some components of the electrochromic device provides improvements in performance and reliability. In some embodiments, all layers of the device are entirely solid and inorganic.
Consistent with above observations and discoveries, the inventors have discovered that formation of the EC-IC-CE stack need not be done in the conventional sequence, EC→IC→CE or CE→IC→EC, but rather an ion conducting electronically insulating region, serving as an IC layer, can be formed after formation of the electrochromic layer and the counter electrode layer. That is, the EC-CE (or CE-EC) stack is formed first, then an interfacial region serving some purposes of an IC layer is formed between the EC and CE layers using components of one or both of the EC and CE layers at the interface of the layers. Methods of the invention not only reduce fabrication complexity and expense by eliminating one or more process steps, but provide devices showing improved performance characteristics.
Thus, one aspect of the invention is a method of fabricating an electrochromic device, the method including: forming an electrochromic layer including an electrochromic material; forming a counter electrode layer in contact with the electrochromic layer without first providing an ion conducting electronically insulating layer between the electrochromic layer and the counter electrode layer; and forming an interfacial region between the electrochromic layer and the counter electrode layer, wherein said interfacial region is substantially ion conducting and substantially electronically insulating. The electrochromic layer and counter electrode layer are typically, but not necessarily, made of one or more materials that are more electronically conductive than the interfacial region but may have some electronically resistive character. The interfacial region can contain component materials of the EC layer and/or the CE layer, and in some embodiments, the EC and CE layers contain component materials of the interfacial region. In one embodiment, the electrochromic layer includes WO3. In some embodiments, the EC layer includes WO3, the CE layer includes nickel tungsten oxide (NiWO), and the IC layer includes lithium tungstate (Li2WO4).
Heating may be applied during deposition of at least a portion of the electrochromic layer. In one embodiment, where the EC layer includes WO3, heating is applied after each of a series of depositions via sputtering in order to form an EC layer with a substantially polycrystalline microstructure. In one embodiment, the electrochromic layer is between about 300 nm and about 600 nm thick, but the thickness may vary depending upon the desired outcome which contemplates formation of the interfacial region after deposition of the EC-CE stack. In some embodiments, the WO3 is substantially polycrystalline. In some embodiments, an oxygen rich layer of WO3 can be used as a precursor to the interfacial region. In other embodiments the WO3 layer is a graded layer with varying concentrations of oxygen in the layer. In some embodiments, lithium is a preferred ion species for driving the electrochromic transitions, and stack or layer lithiation protocols are described. Specifics of the formation parameters and layer characteristics are described in more detail below.
Another aspect of the invention is a method of fabricating an electrochromic device, the method including: (a) forming either an electrochromic layer including an electrochromic material or a counter electrode layer including a counter electrode material; (b) forming an intermediate layer over the electrochromic layer or the counter electrode layer, where the intermediate layer includes an oxygen rich form of at least one of the electrochromic material, the counter electrode material and an additional material, where the additional material includes distinct electrochromic and/or counter electrode material, the intermediate layer not substantially electronically insulating; (c) forming the other of the electrochromic layer and the counter electrode layer; and (d) allowing at least a portion of the intermediate layer to become substantially electronically insulating and substantially ion conducting. Specifics of the formation parameters and layer characteristics for this method are also described in more detail below.
Another aspect of the invention is an apparatus for fabricating an electrochromic device, including: an integrated deposition system including: (i) a first deposition station containing a material source configured to deposit an electrochromic layer including an electrochromic material; and (ii) a second deposition station configured to deposit a counter electrode layer including a counter electrode material; and a controller containing program instructions for passing the substrate through the first and second deposition stations in a manner that sequentially deposits a stack on the substrate, the stack having an intermediate layer sandwiched in between the electrochromic layer and the counter electrode layer; wherein either or both of the first deposition station and the second deposition station are also configured to deposit the intermediate layer over the electrochromic layer or the counter electrode layer, and where the intermediate layer includes an oxygen rich form of the electrochromic material or the counter electrode material and where the first and second deposition stations are interconnected in series and operable to pass a substrate from one station to the next without exposing the substrate to an external environment. In one embodiment, apparatus of the invention are operable to pass the substrate from one station to the next without breaking vacuum and may include one or more lithiation stations operable to deposit lithium from a lithium-containing material source on one or more layers of the electrochromic device. In one embodiment, apparatus of the invention are operable to deposit the electrochromic stack on an architectural glass substrate. Apparatus of the invention need not have a separate target for fabrication of an ion conducting layer.
Another aspect of the invention is an electrochromic device including: (a) an electrochromic layer including an electrochromic material; (b) a counter electrode layer including a counter electrode material; and (c) an interfacial region between the electrochromic layer and the counter electrode layer, wherein the interfacial region includes an electronically insulating ion conducting material and at least one of the electrochromic material, the counter electrode material and an additional material, where the additional material includes distinct electrochromic and/or counter electrode material. In some embodiments the additional material is not included; in these embodiments the interfacial region includes at least one of the electrochromic material and the counter electrode material. Variations in the composition and morphology and/or microstructure of the interfacial region are described in more detail herein. Electrochromic devices described herein can be incorporated into windows, in one embodiment, architectural glass scale windows.
These and other features and advantages of the invention will be described in further detail below, with reference to the associated drawings.